The present invention relates to a brake unit incorporating a hydraulic boosting device to generate a brake pressure in a vehicular brake system having a booster unit connected to hydraulic auxiliary energy, the booster unit being controllable by a brake pedal via a control piston for driving a master brake cylinder unit for the purpose of supply of brake pressure to the wheel brake cylinders through at least one hydraulic line connected to an output of the master brake cylinder unit, with a bypass line being provided for the booster piston, which line is actuatable by the hydraulic auxiliary energy and constructed to preliminarily feed sufficient brake pressure to the wheel brake cylinders to overcome the brake clearance. The bypass line is connected to the drive pressure or working chamber for the booster piston and includes a valve device which will close the hydraulic bypass line after the preliminary feed of brake pressure to the wheel brake cylinders.
In a known brake unit of the above-mentioned type, for example, as described in my copending U.S. application Ser. No. 144,072, filed Apr. 28, 1980, now U.S. Pat. No. 4,341,076, issued July 27, 1982 the bypass line of the booster piston leading away from the drive or working side of the booster unit communicates with a shut-off transmission pressure chamber which is formed between the booster piston and a working piston disposed downstream from the booster piston, the working piston on its part acting on the master brake cylinder piston via a push rod. As is conventional practice, the master cylinder piston displaces pressure fluid during a braking action via at least one hydraulic line into the brake cylinder of the individual wheels of an automotive vehicle. During the preliminary feed of brake pressure to the wheel brake cylinders (to overcome the brake clearance), when the bypass line is open, the booster piston is automatically held substantially in its desired initial position and the working piston actuates the push rod in response to the bypassed brake pressure. During subsequent braking action, after the bypass line is closed, the booster piston is moved in the direction of the master brake cylinder corresponding to the actuating force applied by an operator through the brake pedal and assisted by the auxiliary energy and actuates the working piston as well as the master cylinder piston push rod via the shut-off transmission pressure chamber. The transmission pressure chamber of this prior art requires the formation and provision of an interposed working piston and an additional push rod. Moreover, the booster piston must incorporate a radial breather bore which makes its construction complicated and is detrimental to its overall length.